1. Field of the Invention
The present invention relates to hearing aids. The invention, more specifically, relates to hearing aids comprising means for dynamic modification of the input signals. The invention further relates to a method of processing signals in a hearing aid.
2. The Prior Art
In a modern hearing aid, amplification gain is set according to a prescription to accommodate the user's hearing loss, and is dynamically modified depending on the instantaneous sound level. In state-of-the-art hearing aids this is performed in respective frequency bands. The fitting of the hearing aid aims to confine the level of reproduced sounds between the hearing threshold level, HTL, which constitutes the lower limit, and the upper comfort level, UCL, which constitutes the upper limit. People with a hearing loss have a higher HTL at the frequencies where the hearing loss is present, whereas the UCL is generally less affected by the hearing loss, i.e. very loud sounds still cause approximately the same discomfort as they would to people with normal hearing. Some form of dynamic reduction, or compression, of the output signal from the hearing aid is thus necessary in order to manage larger changes in sound level without exceeding the upper comfort level. This is especially true for transient noises, which may both be very loud, and of very short duration.
Modern hearing aids usually have some way of dynamically compressing or limiting the level of the sound signals in order to keep the amplified sound from the hearing aid output transducer below the UCL. A compressor usually comprises a level detector and an amplifier with a controllable gain level. The level detector provides a measure of the general level of the input signal, e.g. the peak level, the envelope, or some average encountered during a certain time window. The compressors present in contemporary hearing aids usually have their settings optimized during the procedure of fitting the hearing aid to a user's hearing loss for the purpose of reproducing speech faithfully and comprehensible. Other sounds are of course reproduced by the hearing aid as well, but the processing quality of speech signals is paramount.
Speech signals in noise are particularly difficult to understand by a hearing impaired person, and the optimization process thus takes this factor into account when the hearing aid is fitted to the user. Transient noises pose a special problem, as they may be too short for the compressor to react upon and attenuate. Repeated transients may also reduce the amplification of the hearing aid considerably for up to several seconds after the transient sounds are over.
A lot of factors affect the quality of the resulting, gain-reduced signal. The quality of the amplifier, the speed with which the gain is reduced, and the method and the speed of detection of peaks in the input signal all have influence on the perceived quality of the reproduced signal. A slow attack-time, i.e. a low speed in reducing the gain, may result in too much gain when the input level changes from soft to loud. A slow release-time, i.e. a low speed in ramping the gain from the reduced level back up to the normal level, may result in too little gain being applied to soft sounds immediately following loud sounds.
In the present application, a hearing aid should be understood as a small, battery-powered, microelectronic device designed to be worn behind or in the human ear by a hearing-impaired user to alleviate a hearing loss. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription, based on a hearing test resulting in a so-called audiogram, of the initial performance of the hearing-impaired user's unaided hearing, by amplifying frequencies in those parts of the audible frequency range where the user has difficulties in perceiving sounds. A hearing aid comprises one or more microphones, a battery, a microelectronic circuit comprising a signal processor, and an acoustic output transducer. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
The microphone in the hearing aid converts sounds from the surroundings into an analog, electrical signal. The digital signal processor in the hearing aid converts the analog electrical signal from the microphone into a digital form by virtue of an analog-to-digital converter, and subsequent signal processing is carried out in the digital domain. The digital signal is split up into a plurality of frequency bands by a corresponding bank of digital band-pass filters, each band-pass filter processing a respective frequency band. The bank of band-pass filters is usually denoted a band-split filter. The signal processing in each frequency band comprises gain calculation and compression. After processing the signal in the separate frequency bands, the plurality of frequency bands are summed before converting the digital output signal into sound.
Digital hearing aids are thus capable of amplifying a plurality of different frequency bands of the input signal separately and independently and subsequently combining the resulting frequency band signals to form a coherent, audible range of frequencies for reproduction. Part of the amplification process involves a compression algorithm applied for controlling the dynamics of respective frequency bands, and the amplification gain and compressor parameters may be controlled separately for each band in order to tailor the sound reproduction to a specific hearing loss.
The compressors present in contemporary hearing aids usually have their settings optimized during the procedure of fitting the hearing aid to a user's hearing loss for the purpose of reproducing speech faithfully and comprehensibly. Other sounds are of course reproduced by the hearing aid as well, but the processing quality of speech signals is paramount. Speech signals in noise are particularly difficult to understand by a hearing impaired person, and the optimization process thus takes this factor into account when the hearing aid is fitted to the user.
It is well known that sharp transient noises do not agree well with users of hearing aids. The sounds of cutlery and dishes banging together, glasses clinking, paper crumbling, slamming doors or other loud, transient noises not only transcend the threshold of the so-called upper comfort level but are also next to impossible to eliminate by the compressors as generally used, and may be perceived as unbearable noises by the hearing aid user.
A hearing aid capable of reproducing speech in noise in a faithful and comprehensive manner while still attenuating the sharpest transients comfortably is thus desired.
WO-A1-2007031499 discloses a method and an apparatus for attenuating transient noises in a hearing aid. The method involves detecting an envelope curve of the input signal, determining the slope and/or height of an edge of the envelope curve, attenuating the output signal of the hearing device in dependence on the slope and/or height of the edge determined only in the immediate vicinity of the next zero transition of the input signal.
The apparatus, embodied as a hearing device, comprises a detecting device for detecting an envelope curve of an input signal, a data processing device for determining the slope and/or height of the edge of the envelope curve and an attenuating device for attenuating the output signal of the hearing device.
This method has the drawback of having to analyze and extract five different parameters of the input signal, an envelope curve, a slope, a signal peak, an edge presence and a zero transition. The method also has the added complexity of having to calculate the precise attenuation level for each signal peak to be dampened. Furthermore, a zero crossing rule is no guarantee that clicks are not produced by the system, as a change in the slope at the zero crossing of the waveform due to attenuation may still introduce clicks and artefacts into the reproduced sound. A simpler and faster system for detecting and dealing with fast transient peaks in the input signal in a hearing aid is thus desired.